This invention relates to electrical storage cells utilizing a plurality of facing and contacting plate sets, and, more particularly, to a structure that prolongs the operating lives of such cells.
Rechargeable cells or batteries are electrochemical devices for storing and retaining an electrical charge and later delivering that charge for useful power. Familiar examples of the rechargeable cell are the lead-acid cell used in automobiles, and the nickel-cadmium cell used in various portable electronic devices. Another type of cell having a greater storage capacity for its weight is the nickel oxide pressurized hydrogen cell, known as the nickel-hydrogen cell, which is used in spacecraft applications.
The nickel-hydrogen cell includes a series of active plate sets which store a charge electrochemically and later deliver that charge as a useful current, packaged within a pressure vessel that contains the plate sets and the hydrogen gas that is an essential active component of the cell. Each plate set includes a positive electrode, a negative electrode, and a separator between the two electrodes which is impregnated with an electrolyte. In a typical cell, about 40 plate sets are supported on a core under a light compressive loading, with a gas distribution screen between each plate set and with electrical connector leads extending to each electrode of each plate set. A nickel-hydrogen storage cell delivers current at about 1.3 volts, and a number of the cells are usually connected in series to produce current at the voltage required by the systems of the spacecraft.
A nickel-hydrogen cell used in a satellite is periodically charged by electrical current produced by solar panels on the spacecraft when the satellite is in sunlight, and then later discharged to supply electrical power, when the spacecraft is in shadow or peak electrical power is demanded. A satellite in a low earth orbit may experience up to about 6,000 cycles from sunlight to eclipse conditions per year, with a corresponding number of cycles of charging and discharging the cells. A typical accepted industry design objective is attaining satisfactory operation through 30,000 cycles of charging and discharging, corresponding to an operating life of 5 years for the satellite in low earth orbit, or more years in other orbits where fewer battery cycles are experienced annually.
The preferred positive electrode of the plate set in a nickel-hydrogen battery is made of sintered nickel powder impregnated with the active material nickel oxide. The inventors have observed that the positive electrode gradually swells as the number of cycles increases. By way of illustration of the magnitudes involved, a typical nickel positive electrode is initially about 0.030 inches thick, and each plate set is about 0.042 inches thick. After cycling between fully charged and 80 percent discharged states for 8000 cycles, the positive electrode swells to a thickness of about 0.045 inches, a 50 percent growth in thickness, with a corresponding growth of thickness of the plate set.
Each electrode has an electrode connector lead that provides the electrical connection to the voltage regulator electronics external to the storage cell. The connector leads are tightly packed within the core, and when significantly deformed may cause a short circuit, or one or more of the leads may break. The swelling of the positive electrodes causes the entire plate set to expand along the core, thereby causing the entire plate stack of many plate sets to displace along the core by an amount that is cumulative along the core. Some of the connector leads are potentially subject to large deformations, as much as 0.5 inches in a typical cell. The core itself may be deformed or broken by the stresses thereby created. The large deformations result in short circuits or failures.
The short circuiting and potential failure of the connector leads, and failure of the core, are limiting factors in the life of spacecraft batteries. There exists a need for an approach to avoiding, or at least minimizing, the adverse effects of the swelling observed in the presently used positive electrodes. The present invention fulfills this need, and further provides related advantages.